The activity of tyrosine hydroxylase, the rate-limiting enzyme in the biosynthesis of catecholamines, is dependent upon a reduced unconjugated pteridine, tetrahydrobiopterin. We are actively pursuing enzyme purification procedures for tyrosine hydroxylase using as tissue sources both bovine striata and the adrenergic neuroblastoma, N1E115. Pteridine biosynthesis by rat brain, rat pineal gland, and adrenergic neuroblastoma, is being investigated in an attempt to elucidate the pathway by which pteridines are synthesized. The cellular mechanisms which regulate this synthesis, and the interactions between thryosine hydroxylase and pteridines, are also being investigated.